Sequential Mutations in Notch1 and Fbxw7 in Radiation-Induced Mouse Thymic Lymphomas
Kuang-Yu Jen, Jian-Hua Mao, Allan Balmain. University of California San Francisco, San Francisco, CA; Lawrence Berkeley National Laboratory, Berkeley, CA
Background: T-cell acute lymphoblastic lymphomas (T-ALL) commonly show activating Notch1 mutations as well as mutations or deletions in Fbxw7. However, since Fbxw7 targets Notch1 for degradation, genetic alterations in these genes are expected to be mutually exclusive in lymphomagenesis. Previously, using a radiation-induced Tp53-deficient mouse model for T-ALL, we reported that loss of heterozygosity (LOH) at the Fbxw7 locus occurs frequently in a Tp53-dependent manner. In this study, we determined the frequency and nature of Notch1 mutations in thymic lymphomas from irradiated Tp53+/- and Tp53-/- mice. We also examined Notch1 mutations in Fbxw7+/- mice to determine whether concurrent genetic alterations in these genes are seen in mouse thymic lymphomas.
Design: Tp53+/-, Tp53-/-, Tp53+/- Fbxw7+/-, Tp53-/- Fbxw7+/-, and Tp53+/- F1 hybrid mice (Mus spretus and Mus musculus cross) were exposed to a single dose of 4 Gy ionizing radiation to generate thymic lymphomas. Tumors were collected and sequenced for Notch1 mutations in the heterodimerization (HD) and PEST domains. LOH at the Fbxw7 locus was determined for the F1 hybrid mice by PCR analysis.
Results: Only PEST domain mutations were present; no HD domain mutations were observed. Of the thymic lymphomas from Tp53-/- mice, 39% contain Notch1 mutations; similarly, 42% display Notch1 mutations in tumors from Tp53+/- mice. In contrast, no Notch1 mutations are seen in thymic lymphomas from Tp53+/- Fbxw7+/- or Tp53-/- Fbxw7+/- mice. Of the tumors from F1 hybrid mice, 46% contain Notch1 mutations, and 86% exhibit Fbxw7 LOH. 81% of the F1 hybrid tumors with Notch1 mutations demonstrate concurrent Fbxw7 LOH.
Conclusions: Notch1 PEST domain mutations occur frequently in radiation-induced mouse thymic lymphomas. These mutations are independent of Tp53 status since the same mutations occur with comparable frequency in both Tp53+/- and Tp53-/- mice. Tumors from Fbxw7+/- mice lack Notch1 PEST domain mutations; however, concurrent Fbxw7 deletion and Notch1 PEST domain mutations occur frequently in Fbxw7+/+ mice. These data indicate that Notch1 PEST domain mutations must occur prior to Fbxw7 deletion in a temporal sequence of mutational events.
Monday, March 19, 2012 9:30 AM
Poster Session I Stowell-Orbison/Surgical Pathology/Autopsy Awards Poster Session # 232, Monday Morning